The present invention generally relates to formation analysis and more particularly to identifying clay types and properties based on NMR porosity measurements using user-adjusted modeling parameters.
The ability to differentiate between fluid types is one of the main concerns in the examination of the petrophysical properties of a geologic formation. For example, in the search for oil it is important to separate signals due to producible hydrocarbons from bound ones, as well as from the signal contribution of brine, which is a fluid phase of little interest. Extremely valuable is also the capability to distinguish among clay-bound water, capillary-bound water, movable water, gas, light oil, medium oil, and heavy oil.
In this regard, it is desirable to understand the structure and properties of the geological formation. A significant aid in this evaluation is the use of wireline logging and/or logging-while-drilling (LWD) measurements of the formation surrounding a borehole (referred to collectively as xe2x80x9clogsxe2x80x9d or xe2x80x9clog measurementsxe2x80x9d). Typically, one or more logging tools are lowered into the borehole and the tool readings or measurement logs are recorded as the tools traverse the borehole. These measurement logs are used to infer the desired formation properties.
The hydrocarbon production potential of a subsurface formation is described in terms of a set of xe2x80x9cpetrophysical properties.xe2x80x9d Such properties may include the lithology or the rock type, e.g., amount of sand, shale, limestone, or more detailed mineralogical description; the porosity or fraction of the rock that is void or pore space; the fluid saturations or fractions of the pore space occupied by oil, water and gas, and others. Wireline logging tools do not directly measure petrophysical properties, they measure xe2x80x9clog propertiesxe2x80x9d, for example, bulk density, electrical resistivity, acoustic velocity, or nuclear magnetic resonance (NMR) decay. Log properties are related to the petrophysical properties via mathematical or statistical relations, which are generally known in the art. In practice, frequently several different logging tools are combined and used simultaneously to obtain an integrated set of measurements. Thus, different tools may be used to obtain information about the same set of formation properties using different techniques, or different tools may be used to obtain information about different formation properties. In order to make optimal use of the measurement results from different tools, in practice their responses to known formations are modeled and, the model responses are compared to actual logs. The error signal generated in the process serves to improve the parameter estimates of the models and ultimately to provide an understanding of the petrophysical properties of the formation.
Several approaches have been proposed in the past to model the structure of geological formations, as well as the ability of different structures to retain fluids. Such models can be extremely valuable in practice. The present application discloses a new and improved nuclear magnetic resonance (NMR) porosity model using adsorbed water content in clay minerals. The proposed model can be used among other purposes for clay typing in formation evaluation.
NMR logging has proved very useful in formation evaluation. NMR logging tools known in the art include, for example, the centralized MRIL(copyright) tool made by NUMAR Corporation, a Halliburton company, and the sidewall CMR tool made by Schlumberger. The MRIL(copyright) tool is described, for example, in U.S. Pat. No. 4,710,713 to Taicher et al. and in various other publications including: xe2x80x9cSpin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination,xe2x80x9d by Miller, Paltiel, Gillen, Granot and Bouton, SPE 20561, 65th Annual Technical Conference of the SPE, New Orleans, La., Sep. 23-26, 1990; xe2x80x9cImproved Log Quality With a Dual-Frequency Pulsed NMR Tool,xe2x80x9d by Chandler, Drack, Miller and Prammer, SPE 28365, 69th Annual Technical Conference of the SPE, New Orleans, La., Sep. 25-28, 1994. Certain details of the structure and the use of the MRIL (copyright) tool, as well as the interpretation of various measurement parameters are also discussed in U.S. Pat. Nos. 4,717,876; 4,717,877; 4,717,878; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115, 5,557,200; 5,696,448; 5,936,405, 6,005,389 and 6,023,164. The structure and operation of the Schlumberger CMR tool is described, for example, in U.S. Pat. Nos. 4,939,648; 5,055,787 and 5,055,788 and further in xe2x80x9cNovel NMR Apparatus for Investigating an External Sample,xe2x80x9d by Kleinberg, Sezginer and Griffin, J. Magn. Reson. 97, 466-485, 1992. The content of the above patents is hereby expressly incorporated by reference for all purposes, and all non-patent references are incorporated by reference for background.
Technology advances have made it possible to reduce the inter echo spacing of the downhole NMR logging tools such that these tools are able to measure very fast (for example, less than 1 ms) relaxing components of the subsurface rocks. See Prammer et.al., (1996). In most cases, these fast T2 relaxation times are ascribed to the water bound in the clay mineral component of the rocks. As a result, the petroleum industry""s interest in the application of NMR technology to aid in shaly sand evaluation has grown considerably. With reference to the list provided at the end of this section, useful prior art includes the publications by Prammer et. al., (1996); Allen et. al., (1998); Matteson et. al., 1998; Chitale et. al., (1999). However, until recently it remained unclear whether or not the water adsorbed on the clay surface (the so called clay-bound water in petrophysics) is detectable by NMR. This paper presents results of a systematic laboratory NMR characterization of pure montmorillonite clay with an objective to clearly document the NMR signature of the water adsorbed on the clay surface. This will help establish a physical basis for the application of NMR in shaly sand formation evaluation.
NMR relaxometry experiments on clays and clayey rocks have shown that the adsorbed water on the surface of clays (or the so called clay-bound water in petrophysics) is fully represented in the NMR T2 distribution. Accordingly, it is desirable to provide NMR characterization of different clays for use in the interpretation of nuclear (i.e., density and thermal neutron) and NMR porosity logs acquired from shaly sand reservoirs.
Cross plots, overlays and modeling of the log responses with respect to the formation lithology are common techniques in the evaluation of density and neutron porosity logs. Bulk density and neutron porosity values for wet and dry clays are required inputs for such evaluation. Compilations of nuclear logging parameters for various sedimentary minerals, including those for wet- and dry clay minerals have been published in the prior art. See, Edmundson and Raymer, (1979), Ellis et. al., (1994), and wireline logging service companies, such as Halliburton, (1991) and Schlumberger, (1994). Bulk density and neutron porosity values for water-wet montmorillonites published in these compilations are based upon chemical analyses of the clay or theoretical derivations of chemical formulae that include certain fixed number of water molecules per unit cell of montmorillonite. In one aspect, the methods of this invention are used to refine the above wet-clay parameters based on new insights obtained from an NMR study of the physical characteristics of the adsorbed water on montmorillonite surface.
NMR characterization of clays, and more specifically montmorillonites, combined with other spectroscopic data (Touret et. al., 1990) can offer petrophysically meaningful values for the quantity of water associated with montmorillonite occurring in the sedimentary rocks. Deeper understanding of the mechanism of water retention by clays, such as montmorillonite, and of the geometry of the space occupied by the clay-bound water provide a physical basis to quantify the adsorbed water in clays irrespective of their mode of occurrence or morphology, and therefore is desirable in practice.
Details of the methods and techniques in accordance with the present invention are provided below. The interested reader is directed for additional background information to the disclosure of the following references, which are incorporated herein by reference for background. For simplicity, in the following disclosure only the first author and year of publication are provided.
Berend, I., Cases, J. M., Francois, M., Uriot, J. P., Michot, L., Masion, A., and Thomas, F. (1995) xe2x80x9cMechanism of adsorption and desorption of water vapor by homoionic montmorillonites:2: The Li, Na, K, and Cs exchangeable formsxe2x80x9d: Clays and Clay Minerals, 43, p. 324-364.
Chitale, D. V., Day, P. I. and Coates, G. R. (1999) xe2x80x9cPetrophysical significance of Laboratory NMR and Petrographic Investigation on a Shaly-sand Corexe2x80x9d: SPE 56765 Presented at the 1999 SPE-ATCE, Houston, October 3-6, 5 p.
Edmundson, H. and Raymer, L. L. (1979) xe2x80x9cRadioactive logging parameters for common mineralsxe2x80x9d: Presented at the 20th Annual Symposium of the SPWLA, June 3-6, paper O, p.1-20.
Ellis, D., Howard, J., Flaum, C., McKeon, D., Scott, H., Serra, O. and Simmons, G. (1994) xe2x80x9cMineral logging parameters: Nuclear and acousticxe2x80x9d: Petrophysics, SPE Reprints Series No.39, p. 52-66.
Grim, R. E. (1968) xe2x80x9cClay Mineralogy, 2nd Ed.xe2x80x9d: McGraw Hill Book Co., New York, p.250-270.
Guven, N., (1992) xe2x80x9cMolecular Aspects of Clay Water Interactionsxe2x80x9d in Clay-Water Interface and its Rheological Implications; CMS Workshop Lectures, Volume 4; N. Guven and R. M. Pollastro Editors, p. 2-79.
Halliburton Logging Services Log Interpretation Charts (1991), p. APP4a.
Johnston, C. T., Sposito, G., and Erickson, C. (1992) xe2x80x9cVibrational probe studies of water interactions with montmotillonitexe2x80x9d: Clays and Clay Minerals, 50, p.722-730.
Low, P. F. (1980) xe2x80x9cThe swelling of clay: II. Montmorillonitesxe2x80x9d: Soil Sci. Soc. Amer. J., 44, p. 667-676.
Prost, R. (1976) xe2x80x9cInteractions between adsorbed water molecules and the structures of clay mineralsxe2x80x9d: Hydration mechanisms of smectites: in Proc. Intl. Clay Conf. 1975, S. W. Bailey, ed., Applied Publishing Ltd., Wilmette, Ill., p. 351-360.
Prost, R., Koutit, T., Benchara, A., and Huard, E. (1998) xe2x80x9cState and location of water adsorbed on clay minerals: Consequences of the hydration and swellingxe2x80x94shrinkage phenomenaxe2x80x9d: Clays and Clay Minerals, 46, p. 117-131.
Schlumberger Log Interpretation Charts (1994), p. B-6.
Sposito, G. and Prost, R. (1982) xe2x80x9cStructure of water adsorbed in smectitesxe2x80x9d: Chem. Rev., 82, p. 553-573.
Touret, O., Pons, C. H., Tessier, D. and Tardy, Y. (1990) xe2x80x9cEtude de la repartition de l""eau dans des argiles sature""es Mg 2+ aux fortes teneurs en eauxe2x80x9d: Clay Minerals, 25, p. 217-234.
Van Olphen, H. (1965) xe2x80x9cThermodynamics of interlayer adsorption of water in clays: Sodium vermicullitexe2x80x9d: J. Colloidal Sci., 20, p. 822-837.
The limitations of current formation evaluation techniques outlined above are addressed by the present invention, which derives a set of clay parameters and uses these parameters along with measured logs and knowledge of the associated tool response models to provide improved estimates of the properties of clay materials and other earth formation attributes.
The invention is based in part on the determination that adsorbed water on the surface of water-saturated clays, such as montmorillonites, (the so-called clay-bound water in petrophysics) is fully represented in the NMR T2 distribution. This determination finds support in the good match between the NMR-measured and the actual water content of wet clays. In such clays the surfacially adsorbed water is found to coexist with the water occupying the inter-aggregate clay pores, analogous to the condition of water-wet sedimentary clays in the subsurface. These observations further confirm the suitability of NMR techniques for total porosity measurement.
Experiments confirmed that the NMR methods in one aspect of this invention are capable of detecting the adsorbed water on the clay surface even when that is the only form of water present. For montmorillonite this phase of water almost entirely resides on the internal surface of the clay, whose geometry does not change within the hydrated clay. Hence, it was observed that the NMR T2 distributions obtained from these clays are uni-modal. In addition, it was observed that the T2 relaxation is linearly related to the volume of water of hydration V and surface S in accordance with the relationship: 1/T2=pS/V.
In another aspect of the invention, it was observed that NMR T2 distributions obtained from clays, such as water-saturated montmorillonites, are bi-modal because they represent water relaxing in pore spaces with two different geometries. Faster T2 times, which for montmorillonite cluster around 1 ms, are a measure of the water in the hydration shells of exchangeable cations in the interlamellar space of clays. The T2 times slower than 1 ms, on the other hand, represent water relaxing in the inter-aggregate pores of the water-saturated clays. Extrapolation of the hydration data and the NMR T2 distribution combined with the other published spectroscopic data suggest that the water adsorbed on the clay surface in water-saturated montmorillonites amounts to about 500 mg per dry g of clay.
Accordingly, in one aspect of this invention, the use of NMR offers a new approach to differentiating between the surficially adsorbed water in clays, such as montmorillonite, from the interstitial pore water in rocks. As noted, NMR characterization suggests that the threshold level of adsorbed water in this clay is 500 mg/g of dry clay. This parameter is believed to be an intrinsic property of water-wet montmorillonite. A new characteristic parameter, called wetness-clay, is proposed in accordance with the present invention and is defined as the ratio of volume of adsorbed water divided by the volume of wet clay. This parameter constitutes an intrinsic property of clays, and its computed value for montmorillonite is 0.57.
Based on the NMR methods of this invention, the wet clay bulk density values for clays can be corrected. For example based on the NMR measurements, the 2.12 g/cc value prevalent in the literature on log interpretation for montmorillonite has been revised. The wet clay density computed in accordance with this invention for montmorillonite is 1.7 g/cc, which value translates to an apparent porosity of 58% for a standard sandstone matrix. Further, it is proposed to refine the thermal neutron porosity log parameter for water-wet montmorillonite to 70%, considering that the neutron porosity is a composite bulk response from water as also the hydroxyls in clays.
Based on sedimentary petrological considerations and the NMR T2 characteristics of the water-saturated montmorillonites it is postulated that the above values of wetness, bulk density and thermal neutron porosity for this clay remain constant unless the clay undergoes mineralogical transformation.
In another aspect, the present invention makes use of the concept of the wetness clay parameter designating the fraction of the characteristic volume of adsorbed water in a given clay mineral, which is proportional to the volume of that clay mineral. An NMR porosity model using the characteristic wetness values for various clay minerals is then applied in a method of clay typing primarily for use in shaly sand log analysis. The system and method in this aspect of the invention is based on constrained inversion of a completely determined or overdetermined log data set comprising gamma (GR), bulk density (Rhob) and thermal neutron (Nphi) and NMR logs. These logs have well defined linear responses proportional to the volumetric components of matrix minerals, clay minerals and pore fluids. Using the proposed NMR porosity model one can derive a generic log, which accurately reflects the properties of various clay types.
In particular, in one aspect the invention is a nuclear magnetic resonance (NMR) method for differentiating between adsorbed water and interstitial water content in a clay material, comprising: (a) successively passing the clay material through controlled levels of hydration; (b) performing an NMR experiment on the clay material at each of the controlled levels of hydration; (c) computing T2 relaxation distributions from NMR signals obtained in each experiment; (d) determining the minimum level of hydration of the clay material at which T2 relaxation distribution becomes bi-modal; and (e) computing a measure of adsorbed water in the clay material based on the determined minimum level of hydration.
In specific embodiments the method further comprising one or more of the steps of: computing a measure of the interstitial water content in the clay material based on slow T2 relaxation components in a bi-modal relaxation distribution; repeating steps (a) through (e) for a different clay material; providing a listing of the computed measures of adsorbed water for each different clay material; comparing values of the computed measures of adsorbed water for each different clay material to a corresponding value for such material obtained without the use of NMR experiments; and computing a correction of the value of adsorbed water for a clay material obtained without the use of NMR experiments based on the corresponding value obtained in step (e).
In another aspect, the invention is a method for determining petrophysical properties of geologic formations containing clay materials, comprising: (a) providing a model of the geologic formation, said model comprising a wetness parameter for that fraction of the characteristic volume of adsorbed water in a given mineralogy component, which is proportional to the volume Vclay of that component; (b) providing log data corresponding to the geologic formation; (c) performing constrained inversion of the provided data log using the model of the geologic formation; and (d) determining petrophysical properties of geologic formations based on the provided model and the performed constrained inversion. In a preferred embodiment the method uses at least NMR log data, and at least one of the group of gamma, bulk density and thermal neutron log data.
In another aspect, the invention is a geological formation interpretation system comprising: means for providing a model of a geologic formation, said model comprising a wetness parameter for that fraction of the characteristic volume of adsorbed water in a given mineralogy component, which is proportional to the volume Vclay of that component; means for providing log data corresponding to the geologic formation; processor means performing constrained inversion of the provided data log using the model of the geologic formation; and means for determining petrophysical properties of geologic formations based on the provided model and the performed constrained inversion.
In yet another aspect, the invention is a geological formation interpretation system comprising a specially programmed computer having: a first memory for storing a plurality of measurement logs of a geological formation; a second memory for storing one or more tool response models where each response model predicts a measurement log based on a formation description, at least one of said models comprising a wetness parameter for that fraction of the characteristic volume of adsorbed water in a given mineralogy component, which is proportional to the volume Vclay of that component; a processor operatively connected with said first and second memories, performing constrained optimization of the formation description parameters based on a comparison of measurement log data in said first memory and tool response data in said second memory; the processor providing humanly readable output indicia of petrophysical properties of the geologic formation based on the models and the performed constrained optimization.